Why does $_ behave differently when in a .ps1 versus .psm1 files?

Question

Suppose you define map_ps in map.ps1:

function map_ps{
    [CmdletBinding()]
    param([parameter(ValueFromPipeline=$true)]$InputObject,$sb,$ArgumentList)
    process{&$sb $ArgumentList}
}

Suppose you also define another function map_psm with identical implementation in a well-formed module called map.psm1:

function map_psm{
    [CmdletBinding()]
    param([parameter(ValueFromPipeline=$true)]$InputObject,$sb,$ArgumentList)
    process{&$sb $ArgumentList}
}

Calling each function with identical parameters does not yield the same result:

PS C:\> 1 | map_ps  -sb {"DollarBar:$_, Arg:$($args[0])"} -ArgumentList 2
DollarBar:1, Arg:2
PS C:\> 1 | map_psm -sb {"DollarBar:$_, Arg:$($args[0])"} -ArgumentList 2
DollarBar:, Arg:2

Why is $_ empty when the function is implement in a .psm1 but it isn't when the function is implemented in a .ps1?

Answer 1

Unless variable declared in global scope, functions/ScriptBlocks can not see variables declared in module different from its own module. As workaround, you can create ScriptBlocks thru [scriptblock]::Create, which create ScriptBlocks not bounded to any particular module:

function FunctionWithoutModule{
    [CmdletBinding()]
    param([parameter(ValueFromPipeline=$true)]$InputObject,$sb,$ArgumentList)
    process{
        $SomeVariable='SomeValue'
        &$sb $ArgumentList
    }
}
$Module=New-Module -ScriptBlock {
    function FunctionWithModule{
        [CmdletBinding()]
        param([parameter(ValueFromPipeline=$true)]$InputObject,$sb,$ArgumentList)
        process{
            $SomeVariable='SomeValue'
            &$sb $ArgumentList
        }
    }
}
$ScriptBlockWithoutModule={"DollarBar:$_, Arg:$($args[0]), SomeVariable:$SomeVariable"}
$ScriptBlockWithModule=$Module.NewBoundScriptBlock($ScriptBlockWithoutModule)
$ScriptBlockNotBoundToModule=[scriptblock]::Create($ScriptBlockWithoutModule)

1|FunctionWithoutModule -sb $ScriptBlockWithoutModule -ArgumentList 2
#DollarBar:1, Arg:2, SomeVariable:SomeValue
1|FunctionWithoutModule -sb $ScriptBlockWithModule -ArgumentList 2
#DollarBar:, Arg:2, SomeVariable:
1|FunctionWithoutModule -sb $ScriptBlockNotBoundToModule -ArgumentList 2
#DollarBar:1, Arg:2, SomeVariable:SomeValue
1|FunctionWithModule -sb $ScriptBlockWithoutModule -ArgumentList 2
#DollarBar:, Arg:2, SomeVariable:
1|FunctionWithModule -sb $ScriptBlockWithModule -ArgumentList 2
#DollarBar:1, Arg:2, SomeVariable:SomeValue
1|FunctionWithModule -sb $ScriptBlockNotBoundToModule -ArgumentList 2
#DollarBar:1, Arg:2, SomeVariable:SomeValue

Answer 2

I think this is a combination of Module scope and the scriptblock. Being in a module changes the way that local variables are used within the scriptblock ($_ is being used here inside a scriptblock to refer to a variable in the caller's scope).

Use GetNewClosure() on the Script Block

function map_psm{
    [CmdletBinding()]
    param([parameter(ValueFromPipeline=$true)]$InputObject,$sb,$ArgumentList)
    process{& $sb.GetNewClosure() $ArgumentList}
}

That should re-evaluate the scriptblock using the current value of the variables.